Process for preparing polymeric materials for application to printed circuits

ABSTRACT

A process for preparing curable, aqueous strippable, polymeric materials for application, in succession, as masks, to a specific area of each of a plurality of printed circuit boards that is to be protected from the stripping and plating processes associated with gold finger plating, during the fabrication thereof, includes forming a coating of substantially identical, uniformly spaced, areas of polymeric material on a continuous length of carrier web, applying a cover coating over the carrier web and the uniformly spaced areas of polymeric material with the cover coating having a width substantially the same as that of the carrier film, and winding up in a roll the continuous length of carrier web and the cover coating with the uniformly spaced polymeric material sandwiched therebetween.

This a divisional of copending application Ser. No. 07/141,815 filed onJan. 11, 1988.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the application of a heat or photosensitivepolymeric material to a printed circuit board, semiconductor wafer,semiconductor lead frame, or electronic packaging.

2. Description of the Prior Art

Prior to electroplating or stripping operations in the manufacture ofprinted circuit boards, it is the current practice in the art topartially cover the surfaces of the circuit boards or other substrateswith a masking tape. The masking tape comprises a foil-like carrier thatis made of a material such as paper or plastic and a coating of anadhesive material applied to one side of the carrier, which carrier maybe self-adhesive or adhered to the circuit board by a pressure or heatactivation process.

Such masking tapes serve the purpose of protecting and therebypreserving intact, during plating and stripping operations,predetermined portions of circuit boards. For example, during theplating of gold on the fingers or edges of a circuit board, the areas,which are not to be stripped of tin/lead and subsequently gold plated,are protected from the stripping and plating baths by a pressuresensitive tape commonly known as "platers tape."

Many problems are encountered in the use of such a tape and tape maskingprocess. These problems include the following:

1. The tape is manually applied. The application of a tape productmanually is cumbersome and non-automatable.

2. The tape is hand-stripped or mechanically removed which tends toproduce defect-causing "nicks" in the printed circuit board.

3. The residual adhesive usually left behind by the pressure sensitivetape must be removed with an organic solvent. Typically, in printedcircuit facilities, several operators are stationed at the end of thegold plating line. Their major function is to manually remove theplaters tape and then apply organic solvents in an effort to remove theadhesive residue. Reliability problems may result if all the residue isnot removed.

4. The organic solvents required to remove the residual adhesive causesionic contamination problems on the surface of the circuit board and arepossibly hazardous to the health of personnel engaged in the handlingthereof. There is a movement in U.S. industry which is lobbying forreducing worker contact with organic solvents like M.E.K., 1,1,1,trichlorethane, industrial alcohols, and methylene chloride because oftheir potentially adverse health effects.

5. Organic solvents such as methylene chloride, used to remove the taperesidue, can also remove or soften the epoxy resin which is on thesurface of the circuit board.

6. Platers tape does not conform well to the circuit trace geometries.As a result, plating and stripping bath chemicals can wick under thetape to cause uneven plating.

7. Plating bath chemicals which wick under the tape can be released insubsequent baths resulting in cross contamination.

8. The adhesive tape process is labor intensive, and therefore,expensive.

Thus, there is a need for improvement in the materials used and themethod of their application to predetermined portions of the surfaces ofcircuit boards, which portions must be protected from the stripping andplating processes associated with gold finger plating. The presentinvention was devised to fill the gap that has existed in the art inthese respects.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved process forproducing masks on printed circuit boards and other substrates whichavoids the aforementioned problems that have been encountered in theprior art.

Another object of the invention is to provide a process for preparing acurable, aqueous strippable, dry film for application, in succession, toa specific area of each of a plurality of circuit boards that are to beprotected from the stripping and plating processes associated with goldfinger plating. The aqueous strippable dry film may also be used toprotect the gold plated fingers during flow solder assembly of thecircuit boards.

A more specific object of the invention is to provide a process forproducing masks on printed circuits by applying a curable dry film ofpolymeric material to the area of the printed circuit to be protected,wherein the improvement comprises:

(a) forming on a carrier web a curable film of polymeric material havingan area corresponding to the area of the printed circuit to beprotected, said dry film of polymeric material being peelable from thecarrier web,

(b) applying the dry film of polymeric material to the printed circuitby conveying the printed circuit beneath the web with the dry film ofpolymeric material facing the printed circuit and in alignmenttherewith;

(c) affixing the dry film of polymeric material to the area of theprinted circuit to be protected by pressing the carrier web and dry filmof polymeric material against the printed circuit by means of anelastomeric pad having a surface region for contacting the carrier web;and

(d) peeling the carrier web away from the printed circuit.

In accomplishing these and other objectives of the invention, there isprovided a process involving die cutting and ladder stripping from acontinuously moving carrier web having a film of heat sensitive orphotopolymeric material formed thereon, thereby to produce a formationon the web comprising a succession of uniformly spaced pieces, islandsor areas of the polymeric film, all of which areas may be of the samesize and correspond to each of the areas of a plurality of printedcircuit portions to be protected.

A particularly advantageous form of such dry film photopolymericmaterial, that is available commercially under the trademark "LAMINAR,"is disclosed in U.S. Pat. No. 3,887,450 entitled "PHOTOPOLYMERIZABLECOMPOSITIONS CONTAINING POLYMERIC BINDING AGENTS," granted on June 3,1975, to Michael N. Gilano and assigned to Morton Thiokol, Inc., theowner of the above-mentioned trademark and the assignee of the presentinvention. This dry film polymeric material is characterized in that itcan be readily developed by means of an alkaline aqueous solution toyield a resist which when exposed to actinic light is impervious toconventional plating solutions but which is aqueous strippable.

As manufactured, the resist is in the form of a thin dry film or coatingof polymeric material on a carrier polyester support web. Althoughtacky, the dry film is "peelable." This makes it possible to employ atypical 3-layer construction for the resist, involving a polyethylenecover film for protection of the dry film and to facilitate handlingthereof.

In accordance with the invention, a film of dry polymeric material isapplied to an associated one of the predetermined printed circuitportions to be protected by conveying the printed circuits beneath thecarrier web with the dry film of polymeric material facing the printedcircuits and moved into alignment with each, in turn. The dry film ofpolymeric material is affixed to each of the areas of the printedcircuits to be protected by pressing the carrier web of dry film ofpolymeric material against the printed circuit, then in alignmenttherewith, by means of an elastomeric pad having a surface region forcontacting the carrier web. The carrier web is peeled away from each ofthe printed circuits as each, in its turn, has had the dry film ofpolymeric material so applied thereto.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages, and specific objects attained by its use,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

With this description of the invention, a detailed description followswith reference being made to the accompanying figures of drawing whichform part of the specification, in which like parts are designated bythe same reference numbers, and of which:

FIG. 1 is an illustration, in perspective view, of apparatus forproducing precision positioned, uniformly spaced, areas, pieces or"islands" of dry film of polymeric material of the same size on acarrier film;

FIG. 2 schematically illustrates an apparatus including the apparatus ofFIG. 1 for processing a commercially available dry film of polymericmaterial in accordance with the invention;

FIGS. 3, 5, 7, 9 and 11 are top plan views and FIGS. 4, 6, 8, 10 and 12are respectively corresponding cross-sectional end views of acommercially available dry film polymeric material at various stages ofprocessing in accordance with the invention;

FIG. 13 is an illustration, in perspective view, of an apparatusembodiment alternative to that shown in FIG. 1 for producing precisionpositioned, separate, uniformly spaced, areas of polymeric material ofthe same size on a carrier film;

FIGS. 14, 16 and 18 and respectively associated FIGS. 15, 17 and 19collectively illustrate a method of affixing the dry film of polymericmaterial to a printed circuit board; and

FIG. 20 is a greatly enlarged perspective view as seen under a scanningelectron microscope of the manner in which the dry film polymericmaterial drapes over the printed circuit board electrically conductiveelements, in sealing contact therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an apparatus 10 for die cutting and stripping dryfilm of polymeric material 12 from an elongated carrier web 14 in theproduction of precision positioned separate, uniformly spaced, pieces,areas, or islands 16 of dry film of the same size on the carrier web 14.The apparatus, as shown, comprises a rotary die 18 and an associatedbacking roller or anvil 20, which are suitably supported for rotation inopposite directions (by means not shown), with peripheral surfacesthereof in close proximity to and engaging adjacent surfaces of the dryfilm polymeric material 12 and carrier web 14, respectively. Turnrollers 22 and 24 suitably mounted for rotation (by means not shown) andengaging the dry film of polymeric material 12 and carrier web 14,respectively, are provided for enabling the dry film waste portion,designated 26, to be stripped away. The waste portion 26, as shown inFIG. 1, is in the form of a ladder.

FIG. 2 schematically illustrates an apparatus, which incorporates thedie cutting and stripping apparatus of FIG. 1, for preparing a curabledry film of polymeric material in accordance with the invention. FIGS. 3through 12 further illustrate the process at various stages thereof.

Referring to FIGS. 3 and 4, the dry film of polymeric material 12 isshown coated on the carrier support web 14 and provided with aprotective cover coating 28 of polyethylene, thereby forming a 3-layercomposite construction designated 30. Such a composite film construction30, as above mentioned, is available commercially. In FIG. 2, thecomposite film material 30, after being unwound from a standard core(not shown), is shown as being passed between turn rollers 32 and 34which are suitably supported for rotation (by means not shown). At theturn rollers 32 and 34 the polyethylene protective coating 28 isstripped from the material 30 and passed over several overhead guiderollers 36. The film material 30 with the polyethylene cover coating 28stripped away, as shown in FIGS. 2, 5 and 6, is then passed between therotary-die 18 and anvil 20 where the dry film polymeric material 12 iscut into separate, uniformly spaced areas 16, as shown in FIGS. 7 and 8.FIGS. 9 and 10 show the film material 30 with the polyethylene covercoating 28 and also the waste dry film portion 26 of polymeric materialstripped away, the portion 26 being stripped away at the turn rollers 22and 24. After passing through the turn rollers 22 and 24 and having thewaste portion 28 of polymeric material stripped away, as seen in FIG. 2,the carrier web 14 with the spaced areas 16 of dry film polymericmaterial thereon passes between turn rollers 38 and 40 where thepolyethylene cover coating 28 is recombined, that is relaminated, withthe carrier web 14 and the spaced areas 16 of the film of polymericmaterial. This reforms the 3-layer construction and encases the spacedareas 16 of dry polymeric film between the carrier web 14 andpolyethylene protective coating 28, as shown in FIGS. 11 and 12.

Further, in accordance with the invention, the relaminated and reformed3-layer film composite construction may be rolled onto a standarddiameter core, boxed in single or multiple rolls, and readied forshipment.

In FIG. 13 there is illustrated, in perspective view, another embodimentof the invention comprising a one-step method for creating resistpatterns on a carrier support web or film backing which can then belaminated to any suitable substance. Referring to FIG. 13, an uncoatedcarrier web or film backing 42, which may be made of polyester, ispassed between a pattern application roller 44 and a backing roller 46with the rollers 44 and 46 engaging directly opposite sides of the web42. Pattern application roller 44 also is in engagement with a resistpickup roller 48 which, in turn, is in contact with a solution/resistreservoir 49. The rollers 44, 46 and 48 are all suitably mounted forrotation on shafts (not shown) which are all positioned in the sameplane, one above the other.

As shown in FIG. 13, the pattern application roller 44 is characterizedin having a plurality of circumferential pattern application surfaces50, 52 and 54 which are of the same circumferential length and areequally spaced apart about the periphery of the roller 44.

In the operation of the embodiment of FIG. 13, rotation of the patternapplication roller 44 into contact with the resist pick-up roller 48 isoperative to transfer liquid polymeric material thereto from thesolution/resist reservoir 49, which liquid polymeric material, in turn,is transferred to the carrier web 42 to form a succession of precisionpositioned and uniformly spaced areas or islands 56 on the carrier web44, all of which areas 56 are of the same size.

Although not shown, it will be understood that a polyethylene cover filmmay be laminated to the carrier web 42 and spaced areas 56 for theprotection of the latter and to facilitate handling thereof.

As those skilled in the art will understand, a succession of equallysized, precision positioned, and uniformly spaced areas or islands of afilm of polymeric material 56 may be formed on a carrier web 42 ofpolyester or other suitable material by other methods such as the knownsilk-screen process and the known rotogravure process.

The manner of affixing the dry film of polymeric material to a portionof a circuit board to be protected is illustrated by reference to FIGS.14-19. Thus, FIG. 14 shows an area of polymeric material 16 on a carrierweb 14, with the carrier web 14, as shown in FIG. 15, extending from afeed roll 58 to a take-up roll 60. Also as shown in FIG. 15 thepolyethylene cover coating 28 is peeled from the feed roll 58 and woundup on a polyethylene take up roll 61. An exposed area of the polymericmaterial 16 is brought into position immediately above, adjacent and inalignment with a portion of a printed circuit board 62 to be protected.The printed circuit board 62 rests on a stationary platen 64.

Positioned above the platen 64 is a vertically movable heated platen 66having an elastomeric pad 68 affixed thereto that provides a surfaceregion for contacting the carrier web 14. The pad 68 is backed by arigid surface 70 and has a shape and size corresponding to the shape andsize of the area of polymeric material 16 to be affixed to the circuitboard 62. The use of an elastomeric pad 68 backed by the rigid surface70 enables the application of uniform pressure and temperature to thecircuit board 62 or other substrate with a regular or irregular surface.By irregular surface is meant a surface including areas of nonplanargeometry, an example of which is the printed circuit electricallyconductive elements that project outwardly from the surface of a printedcircuit board. There is provided a consistent and uniform pressure toirregular surfaces because the elastomeric pad is soft enough,preferably having a durometer in the range of 40-60, to act similarly toa fluid providing equal and opposite pressure to the entire surfaceunder the surface area of the elastomer pad 68, laterally as well as inthe direction of pressure application. As a result of such pressure onthe entire surface under the pad 68, the dry polymeric film material 16is pressed into conforming and sealing relationship with not only thecircuit board 62 but also with the electrically conductive circuitelements being protected, as illustrated in FIG. 20.

Heated platen 66 is movable between an "Up" position as shown in FIGS.15 and 19 and a "Down" position, as shown in FIG. 17, by motive means72. The motive means 72 per se forms no part of the present invention,and therefore, will not further be described herein.

When moved from the "Up" position to the "Down" position, as shown inFIG. 17, and with the film of polymeric material 16 positioned, as shownin FIG. 16, the elastomeric pad 68 carried by the heated platen 64presses the carrier web 14 and thereby the area of polymeric material 16against the printed circuit board 62. This affixes the polymericmaterial to the printed circuit board 62. At the end of a suitable timeinterval, the heated platen 66 is moved by the motive means 72 to the"Up" position, as shown in FIG. 19. The absence of the area 16 in FIG.18 indicates that the area 16 has been fastened to the circuit board 62as a mask.

While the heated platen 66 is held in the "Up" position, the carrier web14 is wound up on the takeup roll 60, thereby peeling the web 14 awayfrom the circuit board 62 and advancing the carrier web 14 to bring thenext area of polymeric material 16 thereon into proper alignment forplacing a protective mask on the next circuit board 62. Following thisor concurrently therewith, the circuit board 62 upon which a protectivemask has been placed, is replaced with such next circuit board 62. Thissets the stage for the beginning of another cycle, as shown in FIG. 15.

Thus, in accordance with the invention, there has been provided a uniqueprocess for preparing a curable, aqueous processable, dry film forapplication to a specific area of a circuit board, that area being theportion of the circuit board which must be protected from the strippingand plating processes associated with gold finger plating. The processhas utility also for protecting the gold plated fingers during flowsolder assembly. The process is particularly suited for tab fingerplating, semiconductor wafer processing and semiconductor lead frameprocesses.

Subject matter disclosed but not claimed in this application isdisclosed and is being claimed in my copending application for U.S.patent application Ser. No. 142,575 and filed on even date herewith.

With this description of the invention in detail, those skilled in theart will appreciate that modifications may be made to the inventionwithout departing from the spirit thereof. Therefore, it is not intendedthat the scope of the invention be limited to the specific embodimentsthat have been illustrated and described. Rather, it is intended thatthe scope of the invention be determined by the scope of the appendedclaims.

What is claimed is:
 1. Curable polymeric material for application toprinted circuit boards comprising:(a) a carrier web, (b) a coating ofsubstantially identical, uniformly spaced areas of polymeric material onsaid carrier web, and (c) a cover coating over the carrier web and theuniformly spaced areas of polymeric material, said cover coating havinga width substantially the same as that of the carrier web, said carrierweb and cover coating being wound up in a roll with said uniformlyspaced areas of polymeric material sandwiched therebetween.
 2. A methodfor preparing photopolymeric material for application to a workpiece inthe fabrication thereof, comprising the steps of:(a) applying a coatingof curable polymeric film on a continuous length of carrier film, (b)applying a cover coating over the polymeric film to form a compositestrip, (c) separating the cover coating from the composite strip, (d)die cutting the polymeric film to form uniformly spaced, substantiallyidentical polymeric film strips along the length of the carrier film,said uniformly spaced strips being surrounded by waste polymeric filmmaterial, (e) stripping away the waste polymeric film material therebyleaving uniformly spaced polymeric film strips on the carrier filmstrip, (f) applying a cover coating over the carrier film and theuniformly spaced polymeric strips, said last mentioned cover coatinghaving a width substantially the same as that of said carrier film, and(g) winding up in a roll the continuous length of carrier film and covercoating with spaced polymeric strips sandwiched therebetween.
 3. Amethod for preparing curable polymeric materials for application to aprinted circuit board in the fabrication thereof, comprising the stepsof:(a) applying a coating of substantially identical, uniformly spaced,areas of polymeric material on a carrier film, (b) applying a covercoating over the carrier film and the uniformly spaced areas ofmaterial, said cover coating having a width substantially the same asthat of the carrier film, and (c) winding up in a roll the continuouslength of carrier film and the cover coating with the uniformly spacedpolymeric material sandwiched therebetween.
 4. A method as defined byclaim 3 wherein the coating of substantially identical, uniformlyspaced, areas of polymeric material is applied in liquid form on thecarrier film.
 5. A method as defined by claim 3 wherein the coating ofsubstantially identical, uniformly spaced, areas of polymeric materialis applied on the carrier film by a dry transfer process.
 6. A method asdefined by claim 3 wherein the coating of substantially identical,uniformly spaced, areas of polymeric material is applied on the carrierfilm by the use of the known silk-screen process.
 7. A method as definedby claim 3 wherein the coating of substantially identical, uniformlyspaced, areas of polymeric material is applied on the carrier film bythe use of the known rotogravure process.
 8. A method of coating islandsof polymeric material on a carrier web to enable precision positioningof the polymeric material relatively to a workpiece, comprising thesteps of:(a) passing the carrier web between a back up roller and apattern application roller having at least one pattern applicationsurface that extends only partially around the periphery thereof, and(b) rotating the pattern application roller surface into contact with arotating roller surface that is operative to transfer liquid polymericmaterial thereto from a reservoir of liquid polymeric material.
 9. Amethod as defined by claim 8 wherein the reservoir of liquid polymericmaterial comprises a tank of liquid polymeric material, and the rotatingroller surface that is operative to transfer liquid polymeric materialto the pattern application roller is disposed in operative engagementwith the reservoir of liquid polymeric material.